denton
Member
4. I know some of you are absolutely wild about using velocity as a pressure sign. There was a test done some time ago on, as I recall, around 10 "identical" rifles. Muzzle velocities with "the same" load varied by about 200 fps. There are fast barrels and there are slow barrels. In my opinion, muzzle velocity is one of the least dependable so called pressure signs. Remember that published muzzle velocities in may cases were obtained from longer pressure barrels.
It's actually one of the best indicators, as long as you're only varying powder charge.
A firearm with geometry on the maximum side will produce lower pressure and correspondingly lower muzzle velocity than one with minimum geometry. If you measure pressure in that "slow barrel", it will be lower, along with the lower MV. So the slower MV correctly predicts lower pressure, and higher MV predicts higher pressure.
The forces acting on a bullet in the barrel are propellant force (pressure x cross sectional area of the bullet), friction, torque (small loss of energy to spinning up the bullet), and engraving force. MV is a result of these, and not anything else I can think of. Friction is pretty constant, as are torque and engraving force for any given bullet. That leaves just the area under the pressure curve. So for any given type of bullet and type and batch of powder powder, the pressure curve tells the entire tale.